cmake: don't set LLVM_COMPILER_IS_GCC_COMPATIBLE when using clang-cl

Tip-of-tree CMake has become clang-cl aware [1]. In this case,
CMAKE_CXX_COMPILER_ID will still be Clang, but MSVC will be true.

[1] See http://cmake.org/gitweb?p=cmake.git;a=commitdiff;h=3d8356d4